Log Message:
Updated png to 1.6.29.

Version 1.6.29beta01 [January 12, 2017]
  Readded "include(GNUInstallDirs)" to CMakeLists.txt (Gianfranco \ 
Costamagna).
  Moved SSE2 optimization code into the main libpng source directory.
    Configure libpng with "configure --enable-intel-sse" or compile
    libpng with "-DPNG_INTEL_SSE" in CPPFLAGS to enable it.
  Simplified conditional compilation in pngvalid.c, for AIX (Michael Felt).

Version 1.6.29beta02 [February 22, 2017]
  Avoid conditional directives that break statements in pngrutil.c (Romero
    Malaquias)
  The contrib/examples/pngtopng.c recovery code was in the wrong "if"
    branches; the comments were correct.
  Added code for PowerPC VSX optimisation (Vadim Barkov).

Version 1.6.29beta03 [March 1, 2017]
  Avoid potential overflow of shift operations in png_do_expand() (Aaron Boxer).
  Change test ZLIB_VERNUM >= 0x1281 to ZLIB_VERNUM >= 0x1290 in pngrutil.c
    because Solaris 11 distributes zlib-1.2.8.f that is older than 1.2.8.1.
  Suppress clang warnings about implicit sign changes in png.c

Version 1.6.29 [March 16, 2017]
  No changes.